1. Field of Invention
This invention pertains generally to foamed polyolefin materials and, more particularly, to a method and composition for use in making a foamed polyethylene material.
2. Related Art
Foamed polyolefin materials are commonly prepared by melting an olefin polymer resin, mixing the heated resin with a blowing agent, extruding the mixture, and activating the blowing agent to expand the extruded product. One such process, which is concerned specifically with the manufacture of a foamed polyolefin sheet, is disclosed in U.S. Pat. No. 5,281,377.
The blowing agents heretofore used in the manufacture of foamed polyolefins have included n-butane, isobutane, chlorofluorocarbons (CFC's) and hydrochlorofluorocarbons (HCFC's). CFC's and HCFC's can now be used only on a limited basis, however, because they deplete the earth's ozone layer. In addition, n-butane and isobutane are volatile organic compounds (VOC's) whose use is also limited because they produce smog.
Heretofore, there have been some attempts to use blowing agents other than the aforementioned in the manufacture of certain foamed products. U.S. Pat. No. 5,288,740, for example, discloses a process for making alkenyl aromatic foam packing bodies wherein carbon dioxide and/or ethane is used as a blowing agent. There is no suggestion in that patent, however, of using either carbon dioxide or ethane as a blowing agent for a foamed polyolefin.
U.S. Pat. No. 5,290,822 mentions the possibility of using ethane as a blowing agent in the manufacture of a thermoplastic polymer which consists of an olefin polymer in combination with polystyrene and an elastomer. However, the preferred blowing agent is said to be isobutane, and no details are given for using any of the other agents which are mentioned.
U.S. Pat. Nos. 4,640,933 and 4,694,027 relate to the manufacture of expanded polyolefin compositions with a blowing agent consisting of isobutane by itself or in combination with a chlorofluorocarbon or a fluorocarbon having from 1 to 5 carbon atoms and a boiling point between -50.degree. C. and +50.degree. C.
U.S. Pat. No. 3,067,147 discloses the production of polyethylene foam with 1,2-dichloro-tetrafluoroethane as a blowing agent. It suggests that other hydrocarbon blowing agents produce foams having non-uniform large cells.
U.S. Pat. No. 3,523,988 discloses a method of making large celled plastic materials with integral skins. It indicates that a wide variety of extrudable thermoplastic materials and blowing agents can be used in the process, including ethane. However, in the only specific examples actually given, the plastic material is polypropylene, and the blowing agent is nitrogen.
Another method of extruding a foamed plastic with an integral hardened sheath is disclosed in U.S. Pat. No. 3,972,970. Once again, a number of plastic materials and blowing agents are mentioned, including polyethylene and ethane, although no details are given for using the two together. Ethane is also included in a group of possible blowing agents for ethylenic polymer foams in U.S. Pat. Nos. 5,340,840 and 5,288,762, although there again no details are given for that combination.
U.S. Pat. No. 5,348,795 discloses a process for making a dimensionally stable, open cell polypropylene foam. It mentions the possibility of including small amounts non-propylenic polymers including polyethylenes in the mixture, and ethane is one of a number of possible blowing agents which are mentioned. No details are given, however, for the use of ethane in the process.